The present invention relates to a motor brake for an air-compressing internal combustion engine.
Motor brakes are used to assist the brake system of a commercial vehicle; these motor brakes comprise a butterfly valve in the engine exhaust. When the motor brake is actuated, the butterfly valve blocks the engine exhaust, so that during the exhaust stroke, the air in the engine cylinder and in an adjoining exhaust manifold is compressed and thus leads to a braking effect. In order to avoid pressure values that are too high, the diameter of the butterfly valve is generally less than that of the exhaust manifold. With such a motor brake, as measured against the effective power of the engine, only very modest braking powers can be achieved that can merely assist the compressed air brake.
To increase the braking power of air-compressing internal combustion engines, DE-OS 30 26 529 discloses providing in the valve drive a telescoping member that controls the effective length of a valve drive linkage to effect an opening movement of an exhaust valve. For this purpose, built into a valve lifter is a piston that is supplied with hydraulic fluid from a pump piston of a hydraulic pump unit and that, via a push rod, can open the exhaust valve beyond the regular opening phase in order to discharge compressed air in the compression stroke, as a result of which the compression work conveyed to the piston is cancelled or nullified. Each piston of the exhaust valves of a multi-cylinder internal combustion engine is connected via a line with its own pump piston of the hydraulic pump unit, which is a single unit. The pump pistons are disposed radially about a cam that rotates synchronously with a cam shaft. This cam can advantageously be disposed upon the cam shaft itself. The cam is embodied in such a way that the exhaust valves respectively open in the compression stroke in order to discharge the compressed air via an exhaust line. Such a unit permits the braking power of the engine to be increased relative to the otherwise conventional throttling in the exhaust line during the course of the exhaust stroke. Nonetheless, even with such a motor brake one is not in the position to raise the braking power to the order of magnitude of the effective power of the engine.
It is therefore an object of the present invention to further develop a motor brake such that its braking power is increased to the order of magnitude of the effective power of the engine.